Solar Energy Materials & Solar Cells 60 (2000) 263}277
Materials for intelligent sun protecting glazing
Arno Seeboth*, Johannes Schneider, AndreH Patzak
WITEGA Applied Materials Research, Richard}Willsta( tter-Str. 12, D-12489 Berlin, Germany
Abstract
The aim of this review is to present the last 10 years of development in sun protection by use
of electrochromic devices, thermotropic gels and polymer dispersed liquid crystalline systems.
Furthermore, the properties of new lyotropic liquid crystalline and thermotropic hydrogels are
discussed. Thermochromic hydrogels, which change their colour by changes in the temperature,
also are presented. The relationship between gel composition, optical behaviour and thermodynamic properties as investigated by di!erential scanning calorimetry (DSC) is also demonstrated. ( 2000 Elsevier Science B.V. All rights reserved.
Keywords: Sun protection; Electrochromic devices; Thermotropic and thermochromic systems; Liquid
crystalline gels; DSC
1. Introduction
Electrochromic and thermotropic organic materials form systems, which react
accordingly to di!erent external in#uences with a reversible colour modi"cation or
clouding. Granqvist reviews the development of these materials in the years before
1990 in detail [1]. In the past decade this topic has gained further signi"cance. The
motivation for all e!orts to develop chromogenic materials is their potential technical
application as electrically adjustable or thermally self-adjusting light and heat "lters
in the external glazing of buildings. These glazing items are frequently called `intelligenta or `smarta windows. Further potential applications are large area displays for
information and tra$c engineering, as well as temperature sensoring applications in
medical technology.
* Corresponding author. Fax: #49-0-30-63922065.
E-mail addresses: [email protected] (A. Seeboth), [email protected] (J.
Schneider), [email protected] (A Patzak)
0927-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 8 7 - 2
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A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
2. Electrochromic materials and polymer dispersed liquid crystalline systems
The development and the application of electrochromic systems (ECD"ElectroChromic Devices) strongly increased in the last years. Over 2000 patents are
already announced in this technology. The advantages of electrochromic materials
are:
f Electric energy consumption only appears during the switching process and with
a low switching voltage (1}5 V).
f ECDs have a grey scale and low polarisation.
f ECDs are always transparent-typical ECDs has an coverage transmissions of
¹
"70}50% and in the fully coloured state of ¹
"25}10% in the
#0-063-%44
#0-063%$
region of visible light.
A glass slide coated with electrochromic materials modi"es its optical parameters in
response to an electrical "eld. It recovers the original characteristics, if the "eld
polarity is reversed. Monk, Mortimer and Rosseinsky described all aspects of electrochromic systems and materials in detail in their monograph from 1995 [2]. For
application in the external glazing of buildings only transparent and non-re#ecting
electrochromic devices are useful. In a typical transparent ECD with "ve layers, the
outside layers are transparent electrodes. The following layers are the optically
switchable layer, electrochromic "lm, and an ion-storing layer. The internal layer is
formed by the electrolyte or ion conductor, which has a high conductivity for small
ions such as H` or Li` but low conductivity for electrons [3]. The most promising
ion conductors are certain immobile solvent polymer systems, ionic glasses and open
channel metal oxide structures such as perovskites [4]. Applying a DC voltage causes
ion migration into the electrochromic layer and leads to its colouring (Fig. 1). On
Fig. 1. Schematic diagram of an ECD with "ve layers in transmittance mode. With electrical "eld feed of
cations or protons takes place into the electrochromic layer.
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
265
reversing the polarity the "lm delivers the ions under simultaneous clari"cation again.
The voltage must be kept low in order to prevent the initiation of electrolytic
processes or reducing mechanisms in the electrolyte layer.
The best-known material commonly used for the transparent electron conductor is
indium-tin-oxide (In O : Sn; ITO). For the electrochromic "lm the transparent
2 3
tungsten oxide WO is particularly suitable. If both electrodes bear an electrochromic
3
layer like in a typical "ve-layered ECD, one oxide layer is present in its reduced form
while the other is oxidised. During the storage of small ions into the crystalline lattice
the frequently blue coloured mixed oxide M WO is formed by a typical cathodic
x
3
reaction (reduction process):
WO (pale yellow/colourless)#xM`#ye~NM WO (blue).
3
x
3
The complementary anodic reaction (oxidation process), eg. for the formation of the
coloured vanadium pentoxide:
M V O (pale blue)!xM`!ye~NV O (brown).
x 2 5
2 5
Thus the complete redox reaction is
WO (pale yellow/colourless)#M V O (pale blue) H
3
x 2 5
"-%!#)%$
M WO (blue) #V O (brown) .
x
3
2 5
#0-063%$
Beside WO the metaloxides NiO, MoO and IrO have gained the most research
3
3
x
interest in the last years.
Four typical kinds of ECD can be di!erentiated:
f ECDs with liquid electrolytes (e.g. LiClO #propylene carbonate#2% H O,
4
2
limited by a primary "lm of WO and an ion-storing iridium oxide layer, "ve layers
3
between two glass slides) [5].
f ECDs with solid inorganic ion conductors of high volume.
f ECDs with solid inorganic ion conductors in the form of thin "lms (e.g. glass coated
on one side with ITO, WO , LiNbO as an ion conductor and V O as ion
3
3
2 5
memory layer) [6].
f ECDs with solid polymer electrolytes [7] (e.g. poly(propylenglycol) poly(methyl
methacrylate) (PMMA)}LiClO as electrolyte in a "ve-layered structure between
4
two glass slides) [8].
ECDs of the latter type are of particular interest for their use in intelligent windows.
In these electrochromic windows the high viscose polymer electrolyte makes it more
di$cult to #ow out if the glass breaks. By Mani and Stevens [9] in the presence of
a poly(propylene glycol)/LiCF SO mixture methylmethacrylate was polymerised to
3 3
a network in order to increase the mechanical stability of the electrolyte. This system
showed no phase separation. All redox active and electrically conductive polymers are
266
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
potentially electrochromic in thin-"lm form. Most of them are aromatic systems
like polyaniline. Polyaniline and some other conductive polymers, are so-called
organic metals and might be understood as real metals, which have some limitations
due to their mesoscopic properties. The organic metal "lms of polyaniline are
transparent (green) and highly conductive even at thickness of only 1 lm. The
application of conducting-polymers as electrochromics has been reviewed by Mastragostino [10] and by Hyodo [11]. Jelle et al. [12] used poly (2-acrylamido-2methylpropane sulfonic acid) as solid polymer electrolytes. The two ITO coated glass
slides, when coated with polyaniline on the side directed to the light and with WO on
3
the other side, blue colouration occurred by complexation of polyaniline with formed
H SO . More than 50% of the incident solar radiation could be held back by
2 4
a further improvement of this system [13]. Likewise polymer electrolytes with complementary electrochromic "lms of polyaniline and WO are investigated by Panero
3
et al. [14].
The most studied organic electrochromics are the viologens or bipyridiliums. For
short alkyl chain length 1,1-dialkyl-4,4-bipyridiliums, both the dication and radical
cation states are soluble in water and any electrochromic device (ECD) using such
bipyridiliums would have the limitation of a low colouring}decolouring e$ciency.
One solution to this problem involves electrostatic binding of bipyridilium dications
into anionic polyelectrolyte "lms [15] and the additional use of longer alkyl chains
(e.g.: 1,1@-diheptyl-4,4@-bipyridilium, heptyl viologen, HV) lowers the solubility in
water. E.g. one approach is to incorporate bipyridiliums into "lms of the sulphonated
per#uorinated polyether Na"ont [16,17].
Goldner et al. [18] examined glass substrates coated only on one side. They also
used crystalline WO as cathodic electrochromic layer, amorphous LiNbO as an ion
3
3
conductor, for the second electrochromic layer In O , Nb O or LiCoO and as
2 3
2 5
2
limiting transparent electrodes ITO or In O . Frand et al. [19] used a mixture from
2 3
LiClO solved in propylene carbonate with 50% PMMA. The formed solid electro4
lytes were highly transparent and thermally stable up to 1603C. A switching time of
only 20 s between 20% transmission (dark blue) and 80% transmission (clear) was
obtained by Andrei et al. [20] with a polymer electrolyte based on polyepoxide,
complexed with LiClO . Passerini et al. [21]. examined proton or lithium-cations
4
conducting gel-like electrolytes with two di!erent electrochromic layers of WO and
3
NiO . Beside WO as electrochromic layer Lampert and Ozer [22] examined the
x
3
cathodic oxides TiO and Nb O , the anodic hydroxides Ni(OH) and Co(OH) as
2
2 5
2
2
well as the oxides V O and CuO . With these compounds di!erent colouring can be
2 5
x
produced. Ni(OH) shows, e.g. a colour change from transparent to bronze/brown. By
2
withdrawal of y Li` ions the pale-yellow Li V O shows a transition to the blue
y 2 5
V O [23].
2 5
The utilisation of photochromic e!ects can also be used in ECDs. Gregg [24]
describes a photoelectrochromic system that was build up by combination of a colour
sensitive solar cell with an electrochromic electrode. A further advancement of
Bechinger and Gregg [25] possesses the typical "ve-layered structure. In place of the
ion memory they use a colour sensitive TiO layer, which serves as semiconducting
2
electrode for the production of a photoelectric voltage, whereby the discolouration of
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
267
the WO -"lm is caused. The electrical voltage needed for switching the window is thus
3
produced only by in#uence of light. With very thin electrolyte layer homogeneous
discolouration can also be attained if only one part of the cell is exposed to light.
ECDs are investigated by many research institutes and companies around the
world. The commercial application of electrochromic systems is not limited for the
glazing of buildings. Also in the future automotive manufacture (windows, glass roofs
and mirrors) and for the aeroplane technology (e.g. cockpit window in aeroplanes)
applications are possible. In fact, mirrors are the most commercially electrochromic
products. In 1991, the Gentex Co. was the "rst to market with a driver side #at
exterior * NVS Night Vision SafetyTM (NVSt) mirror. Further details of the market
situation for ECDs and commercialised chromogenic products are reported by
Lampert [26].
Likewise it is possible to use a polymer matrix with stored liquid crystalline
droplets (Polymer-dispersed liquid crystals * so-called PDLCs) as the optically
active layer. At "rst in the cloudy layer the liquid crystalline molecules are not aligned.
By applying an electric "eld the molecules are aligned parallel and the PDLC layer
becomes transparent (Fig. 2).
Crawford and Doane [27] examined 10 lm thick polymer layers with LC-Droplets
of (1 lm diameter. These could be reoriented in a light-transmittance status by
a voltage of 40 V. For chiral PDLCs with large cholesteric pitch and negative
dielectric anisotropy it was observed that in the centre of the LC droplets uniformly
Fig. 2. Orientation of PDLCs in the electrical "eld. In the case of orientation toward the "eld the system
posses high transparency.
268
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
aligned regions were developed if an electric "eld is switched on. Their radius rises
exponentially with increasing "eld strength. On the other hand the threshold voltage
decreases with an increase of droplet diameter and pitch [28].
Seeboth [29] determined threshold voltage and absorption of PDLC "lms
under in#uence of an electrical "eld on lyophilised peptide layers with formerly
embedded liquid crystals. For a "lm with layer thickness of 30 lm a threshold
voltage around 35 V was determined. Hydrogels are likewise applicable as a
polymer matrix in PDLC systems. Liquid crystals embedded in a hydrogel of poly
(vinyl alcohol) crosslinked with borax was electrically controllable. That PDLC was
stable against electrolytic decomposition, but the required threshold voltage was too
high [30].
3. Thermotropic materials
3.1. Hydrogels and polymer blends
These materials are usually organic compounds and show, in a system depending
temperature range, a reversible and more or less sharp transition from transparent to
cloudy. Glazing based on these materials will be applicable in areas of buildings,
which do not require constant transparency of the glazing, as for instance roof glazing,
glazing in shopping centres or industrial buildings.
The use of thermotropic polymerfoils consisting of polymer blends [31] coated
directly onto the glazing or the use of double glazing "lled with a thermotropic
substance are practicable ways of realisation (Fig. 3).
From our point of view the latter one is the more promising system. A glazing "lled
with thermotropic gel shows a better, well-de"ned transmission-temperature behaviour and is more resistant to exposure than polymerblends. A set of restrictions arise,
without ful"lment an industrial application using this kind of sun "lter must fail:
Fig. 3. Possible Smart windows: (a) polymerblend (&0.2 mm) coated directly onto the glazing. (b) glazing
"lled with a thermotropic polymer gel, thickness of the layer is 1}2 mm.
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
269
f The used substances and systems should be stable against UV-radiation, biologically degradable, inexpensive, available in large quantities, innocuously, with low
or non-#ammability, free of organic solvents, safely manageably and non-freezing.
f The systems should possess a high viscosity in order to distribute the internal
hydrostatic pressure evenly on the glazing and not to run out of the glazing.
f The temperature-dependent transitions of the glazing should take place between
a status of high transparency with a transmission '85% and a clouded, scattering
status (paper white) with a transmission (15%.
f The turbidity should be distributed over the whole area of the glazing items,
without appearance of streaks and with an acceptable rate for the human eye.
f The temperature, at which these transitions occur, should range between 303C and
803C.
f Furthermore, these transitions must be highly reversible and reproducible during
a long period.
Promising systems are polymer-hydrogels [32] which are proved to ful"l not only
most of, but also the most important of these requirements. Polymer-hydrogels are to
date extensively used in pharmaceutical [33] and medical technology [34,35]. Hydrogels are interlaced macromolecules, in which at least one part of the network consists
of hydrophilic groups. These polymer networks can be poured with the appropriate
water content [36]. The swelling of the hydrogel results from cooperation of molecular interactions in aqueous, partial ionic polymer solutions on the one hand, and
#exible characteristics of polymer networks on the other hand. At low temperatures
the macromolecules are mixed as a homogeneous aqueous solution. During the rise in
temperature an aggregation of the polymers or separation of water from the polymer
network occurs. If the phases possess di!erent refractive indices, the consequence is
scattering of light and a visible clouding of the system. Layer thickness within the
range of 1}5 mm commonly applicable for industrial production of "lled glazing.
Even for layer thickness of 1 mm extensive scattering of the incident light with a large
proportion of re#ection occurs [37]. Investigations of gelation [38,39] and of the
swelling behaviour [40] were again strengthened in the last years. This particularly
applies to the phase transitions in non-ionic hydrogels discovered by Tanaka in 1984
[41]. As suitable thermotropic polymer systems mainly water-soluble mixtures or
copolymers from poly(acrylic acid) derivatives as well as systems based on poly (vinyl
alcohol), cellulose ester, polyglycol, poly (vinyl acetal)-resins and polyether are examined. In the years 1991/1993 partially crosslinked copolymers from N, N-dimethylacrylamide and C }C alkyl and alkoxyethyl acrylate were developed by Mueller
1 4
[42}44] (Ciba-Geigy). These hydrogels showed sharp clear-cloudily transitions between 303C and 803C, which were accompanied by shrinking of the gel. In the year
1977 a thermotropic compound of acrylamide polymers and poly (vinyl alcohol) or
poly (vinyl caprolactame) was already developed as a thermotropic component for the
"lling of double glazing by Rullier [45] (Saint Gobain Industries). The earliest attempt
of this type took place between 1950 and 1960 in Germany in the residence of Munich
zoo. A mixture of 5% poly (vinyl methylether) in agar}agar was used for the "lling of
a double glazing. This system however, reduced the transmission insu$ciently and is
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A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
therefore not of practical use. In 1995, a thermotropic system on the basis of poly
(methyl vinylether) solved in water and crosslinked with methylenebisacrylamide was
developed by Chahroudi [46,47] (Suntec Corp.). By addition of divinylspirodioxan
this hydrogel is able to stick well to the "lled materials from poly (vinyl acetate) or
polypropylene. This thermotropic hydrogel is already commercially available as
Cloud GelTM, however with limited success. Investigations on the thermotropic
optical behaviour of this system were initiated by Wilson [48] and by Wittwer et al.
[49]. In a layer with a thickness of 1 mm a change in transmission by 65% was
observed (Fig. 4).
Polymerised ethylenic unsaturated N-substituted lactame, N-vinylsuccinimide as
well as vinylethers and copolymers of these monomers with crosslinking comonomers
were also developed 1995 by KroK ner and Jahns [50] (BASF) for application as
thermotropic gels. The in#uence of di!erent crosslinkers on the homogeneity of
hydrogels on the basis of poly (acrylic acid) was examined in 1997 by Oppermann
et al. [51]. In order to prevent the irreversible sedimentation of polymer units often
accompanying the phase separations and to implement the turbidity of the gel by
a phase transition, Watanabe [52,53] added the amphiphilic poly (oxy propylene-2ether-2-hydroxymethyl-1,3-propanediol) (MW"400) to a gel made from hydroxypropylcellulose and water. The non-ionic amphiphile thereby acts as spacer preventing
a complete phase separation. Meinhardt already tested the use of nonionics in
thermotropic systems in 1987 [54]. The basic polymer of his system was a watersoluble polyether with ethylene-oxide functions mixed with poly (acrylic acid) and
a base. The addition of nonylphenol as a wetting agent also caused here an improved
reversibility of clear-cloudily transitions. The content of wetting agents controlled the
temperature range of these transitions. The gelation achieved by the complexation or
Fig. 4. Spectral transmittance of a Cloud-GelTM sample at di!erent temperatures [49,50].
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
271
crosslinking of aqueous poly (vinyl alcohol) (PVA) with borax is also a well examined
system [55,56], which can be used as a gel basis for the production of thermotropic
systems.
Non-aqueous, thermotropic polymer blends are likewise examined beside the
hydrogels for their applicability as light and heat "lters. Polymer blends are poured
mostly as a "lm from an organic solvent on a glass or polymer substrate or made as
foils. In 1993 Siol et al. [57] (Roehm GmbH) developed an applicable polymer blend
made from chlorinated rubber and polymethacrylates. This polymer blend showed
sharp cloud points between 603C and 1403C depending on the polymer composition.
In the same year Eck et al. [58] developed a thermotropic polymerblend from poly
(propylene oxide), a styrene-hydroxyethyl acrylate copolymer and a trifunctional
cyclic isocyanate as crosslinker. The toluene solution of the polymer blend was poured
to a thermotropic "lm. The temperature range, in which the turbidity occurred, also
was controlled by the degree of crosslinking. By crosslinking the copolymer in the
presence of propylene oxide a semi-interpenetrating network was obtained. A likewise
crosslinked thermotropic polymer network was developed by BASF [59] in 1995. The
dried "lms, formed from a styrene hydroxymethacrylate copolymer and a poly
(propylene oxide) by radical polymerisation, showed reversible turbidity transitions
between 403C and 1303C. The combination of a non-thermotropic polymer matrix
and a higher alkane as a thermotropic component was presented in 1996 by Meinhardt et al. [60]. The polymer matrix was based on a resin from phthalic anhydride,
soy oil, pentaerythrit and a solvent. The "lms obtained after drying clouded themselves between 303C and 403C reversibly.
3.2. Lyotropic liquid crystalline hydrogels
Synthetic lyotropic LC polymers are well examined since the "rst observation of
this phase formation in poly (benzyl glutamate) [61,62] (PBLG), hydroxypropyl
cellulose [63,64] (HPC) and full-aromatic polyamides [65]. Common properties of
these polymers are their good solubility in water and the structural feature of a more
or less rigid polymer backbone (sti! chain of polymer) which enable the formation of
liquid crystalline phases [66]. With an increase in the polymer concentration in the
solvent, e.g. water, these polymers undergo a transition from an isotropic phase to
a polymer solvent system with pronounced long-range order [67].
Our interest applies on the investigation of lyotropic LC-polymer gels, which are
stable over a temperature range as large as possible in the gelled state. We could
implement this, e.g. by a PEG/PVA borax system [68]. PEG forms liquid crystalline
domains in a PVA-borax network, which is to be detected by the formed schlieren
(Fig. 5a) and radial-droplet (Fig. 5b) structures using polarised optical microscopy.
The temperature-induced transition from the optically anisotropic LC-phase into
the isotropic phase could be e!ected in this system without destroying the gel
network. Both the optical and the gel characteristics are strongly dependent on the
molecular weight and the molecular weight distribution of the polymers used here, the
weight ratio of the individual polymers, the total concentration of the polymers and
the degree of crosslinking. The temperature-dependent optical behaviour is well
272
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
Fig. 5. Schlieren texture (a) und droplet texture (b), observed in the liquid crystalline phase between crossed
polarisers.
adapted to the human eye. This is achieved by the fact that transmission modi"cations
up to 85% take place not suddenly but in a quite broad temperature range of 5}15 K.
Thus temperature gradients occurring at the warming up or cooling of large glazings
do not cause abrupt di!erences of the transmission. It is particularly interesting, that
lyotropic gels can posses several maxima and minima of transparency over a wide
temperature range. Thus for an aqueous lyotropic gel of ethoxylized poly (dimethyl
siloxane) (ePS)/PVA-borax [69,70], a transition from cloudy to clear and again too
cloudy can be observed on heating up (Fig. 6). These thermotropic systems can protect
interiors against overheating at elevated temperature and against cooling down at low
temperatures. The reasons for the di!erent optical states are phase transitions in the
lyotropic LC system as well as phase separation at usually higher temperatures.
A further important point is the systematic investigation of the in#uence of salts on
the transmission behaviour of thermotropic hydrogels. In certain polyether/water
mixtures the formation of an anisotropic phase is induced by the addition of salts
[71]. With addition of 1 wt% alkali chloride the polyether/water gel forms an
anisotropic LC-phase between 203C and 283C. This anisotropic phase is detected as
low turbidity of the former transparent gel. On warming up phase separation occurs.
It is well known that the addition of alkali halides and particularly alkali chlorides
lowers the temperature at which the phase separation occurs [72]. In the process of
phase separation salt extracts free water from the polymer network by hydration of
the ions and polar interactions of hydrates with polar molecule fragments occur.
These polar fragments are usually oxygen or nitrogen atoms. We used alkali halides
for our investigations, in order to study their in#uence on the phase separation and on
the formation of the lyotropic LC-phase systematically. We observed that all salts
strongly lower the temperature of the phase separating clear}cloudy transition. With
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
273
Fig. 6. Temperature/transmittance plot for ePS/PVA gels [70]. (h) ePS/PVA 70T 1 : 3 (15% polymer,
1,6% borax), (L) ePS/PVA 70T 1 : 3 (20% polymer, 1,6% borax).
Fig. 7. Salt induced lyotropic LC-phases in polyether/water system [71].
alkali-bromides and -iodides a reduction of this e!ect in the order K'Na'Li was
observed, which correlates with the decrease of the ion radii. On the other hand, the
chlorides show a gradation in the order NaCl'LiCl'KCl, which does not obey
this pattern. In comparison to NaCl and LiCl the LC phase shifts even with KCl
addition from about 213C around 5 K to higher temperatures (Fig. 7). With an
increase in the water content from 20% to 30% an anisotropic LC-phase is formed
without addition of salt, formation of which can be observed in a temperature range
between 173C and 323C.
This temperature range also is maintained with salt addition. Additionally, the
increase of the water content degrades the temperature of the phase separation by
11 K to 443C. This e!ect is even still strengthened by salt addition. In addition to
274
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
alkali halides also the e!ect of carbonates, nitrates and phosphates was examined. We
understand however, predicting the in#uences of salts on the optical behaviour of the
gels are only possible with di$culty.
Phase separations and phase transitions in hydrogels can be followed well by
transmission spectroscopy and also di!erential scanning calorimetry (DSC). Investigations in this direction are however rather the exception [73] but the di!erent types
of water in hydrogels are examined well [74,75]. Due to di!erent heating and cooling
rates a direct comparison of the DSC measurements with the transmission spectra is
only possible conditionally. By the example of the salt-free polyether/water system
with a water content of 30 wt% the occurrence of the anisotropic phase was "rst
observed at 273C [76]. In the DSC appears a quite sharp endothermic peak with
a maximum at 273C (Fig. 8). The two isotropic phases that are present at lower
temperatures could easily be detected in the light microscope under polarised light.
The anisotropic phase begins to rearrange at 363C into an isotropic phase. This
temperature correlates approximately with the temperature range of highest transmission in the UV/VIS spectrum. By further heating to 443C phase separation occurs. At
this temperature a strong decrease of around approximately 90% of the transmission
in the UV/VIS-spectrum is observed. Only the 40 K broad, endothermic rise of the
DSC curve shows, that the phase separation process runs over a large temperature
range and is completed by 843C (¹ ).
$
Fig. 8. UV/VIS-spectra and DSC plot of polyether/water system. ¹ start of formation of the anisotropic
!
phase, ¹ begin of formation of the isotropic phase, ¹ begin of phase separation, ¹ end of phase
"
#
$
separation.
A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
275
Fig. 9. Thermochromic behaviour of cresolol red embedded in a PVA/borax network at pH 8.5. UV/VISspektra (d"1 cm) at di!erent temperatures.
4. Thermochromic hydrogels
Gels with reversible colour changes, e.g. thermochromic gels are rarely described
[77,78]. For many applications, e.g. dye sensors [79], large area displays with higher
information density or intelligent windows, it is interesting to produce transparent
hydrogels which in closed systems exhibit thermochromic behaviour in the practical
relevant temperature range of 10}803C. Recently we have found [80], that pHsensitive dyes, like the Reichardt-dye E (30), [81,82] or cresol red, embedded in an
T
aqueous polyvinyl alcohol/borax/surfactant gel network can reversibly change their
colour. The novel hydrogels are fully transparent at the investigated temperature
range of 10}803C. In case of E (30) at pH"8.5 the colour changes gradually from
T
colourless at 103C to a deep violet at 803C. Similarly for cresol red a change from
yellow to wine-red was observed. The corresponding UV/VIS-absorption for cresol
red show an increase of the absorption maximum intensity at j"581 nm while the
intensity of the second maximum at j"419 nm is reduced with raising temperature
(Fig. 9). An explanation for the observed reversible colour changes in the closed
hydrogel systems is given by a temperature-induced shift of the equilibrium between
the phenolate and the phenol form of the dye molecules in the micro-environment of
the gel network.
Acknowledgements
This work was supported by the Federal Ministry of Education, Science, Research
and Technology (BMBF), Germany - Projekt No. 0329820B.
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A. Seeboth et al. / Solar Energy Materials & Solar Cells 60 (2000) 263}277
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